Exploring the Parameter Space of Colloidal High-Entropy Alloy Synthesis Methods

High-Entropy Alloy (HEA) nanoparticles have been subject to increasing interest as prospective catalyst materials. The potential of HEAs stem from their multi-component nature, allowing the tailoring of catalytic properties by controlling their structure and composition. However, highly controllable synthesis methods of HEA nanoparticles remain scarce. To address this issue, we present a systematic study of the synthesis parameter space in a simple colloidal synthesis approach where RhIrPtPdRu nanoparticles were synthesised in triethylene glycol. We show that ultra-small (1-3 nm), single-phase HEA nanoparticles can be easily synthesised through a hot-injection synthesis approach. In contrast, multiple phases are observed when a one-pot synthesis is carried out. We also show that the particle size and morphologies relate to the choice of metal precursor counterion, and further investigate the dependence of metal reduction on the reaction temperature. Additionally, we present the synthesis of mixed noble/non-noble-metal IrPdPtRuNi and IrPdPtRuCo HEA nanoparticles. By carrying out reduction kinetics experiments, we relate phase separation and compositional gradients of some of the synthesised samples to the large differences in the reduction rates of the reacting metals. The individual metal reduction rates are found to vary depending on which other elements are present during particle formation. This emphasises the importance of understanding the chemical landscape during reaction for the controlled synthesis of HEA nanoparticles.